Since polyesters as typified by polyethylene terephthalate (PET) or the like is characterized by excellent transparency, mechanical properties, melt stability, aroma retainability, recyclability and the like, they are currently widely used for various packaging materials such as films, sheets and hollow containers.
Since the gas barrier properties of polyesters against oxygen, carbon dioxide gas or the like are not always sufficient, improvement of the gas barrier property has been attempted by melt-blending a polyester with polymetaxylene adipamide (MXD6) which has a high gas barrier property and whose molding temperature is close to PET. However, a compact prepared by using a resin composition containing a polyester and polymetaxylene adipamide has inadequate transparency, and thus there is a restriction on the use thereof for use that requires high transparency.
Conventionally, a strand cut system is employed in a method for granulating a resin, where a resin is melt-kneaded in an extruder, extruded into a strand shape from the die surface of the extruder, cooled by water cooling or air cooling, and then cut with a cutter to obtain granules (pellets). If the melt viscosity of the resin is too low, however, the resin cannot be molded into a strand shape, and thus cannot be granulated by this method.
As another method, an underwater hot-cut system is known, in which a resin is melt-kneaded in an extruder, and then the molten resin is extruded from the die surface of the extruder into water, and cut with a rotary blade near the die surface. However, since the die surface is directly exposed in warm water, heat loss is significant. Thus, the temperature of the die needs to be set at a high temperature, which tends to cause a problem of resin burning at the die part.
If a high-viscosity resin is used, an air hot-cut system can be employed in which a resin is melt-kneaded in an extruder, then allowed to flow out from the die of the extruder, and cut with a rotating knife near the die surface (Patent Literature 2: Japanese Unexamined Patent Application Publication No. Heisei 7-9439). This method, however, is difficult to be applied to a low-viscosity resin, and thus there has been a need for a good method for granulating a low-viscosity resin with less variation in the shape.
As a method for granulating a high-viscosity resin, a method is proposed in which a resin is melted by heating in an extruder, supplied into recesses provided in a main drum for solidification by cooling, and the resulting granules is taken out by vacuum suction to obtain granules (Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2003-33913). This publication describes, in the section under “Prior Art”, that if a liquid material in the molten state has a low viscosity, droplets can be dropped continuously onto a cooling belt for solidification by cooling so that granules can easily be produced by removing them off from the cooling belt. However, even if droplets of a resin in the molten state can be dropped continuously onto a cooling belt, there were some cases where the granules were practically hard to remove from the cooling belt without a special device. In particular, when a polyamide is to be granulated, adhesiveness between the polyamide and the metal of the cooling belt may be too high such that it is hard to remove the granulated polyamide from the cooling belt, which leads to problems of damaging the granulated pellets or causing cobwebbing or the like upon removal.